"Web Callback". Problems!
 

There Is a delay of a sound. How it to correct?

I believe the answer is that when using web callback, the call data between the two parties is routed through an American proxy server, whereas when you directly call or three-way call the parties from a softphone or ATA, there is no relaying of voice data as there is with web callback.

At this point there is no meaningful way to improve latency with web callback, but Voxalot is planning on adding more servers around the globe which might be closer to you and thus improve your connection. If web callback is making too much delay right now for you, the best option might be to just not use it until new servers come online.

Whether you plan to establish proxy server in the Europe?

Yes we are currently considering either the Netherlands or Germany as a location. ETA is unknown at this stage.

Thanks, we shall wait.